How do I design a microstrip to coplanar waveguide transition for a probe measurement?
Probe Launch Transition Design
On-wafer probing uses GSG (ground-signal-ground) probes to make measurements on planar circuits without connectors. Microstrip circuits need a transition structure that converts the bottom-ground microstrip field distribution to a coplanar ground structure accessible by the probe tips. This transition must maintain 50 Ω impedance throughout its length to avoid introducing measurement artifacts.
| Parameter | Semi-Rigid | Conformable | Flexible |
|---|---|---|---|
| Loss (dB/m at 10 GHz) | 0.8-2.5 | 1.0-3.0 | 1.5-5.0 |
| Phase Stability | Excellent | Good | Fair |
| Bend Radius | Fixed after forming | Hand-formable | Continuous flex OK |
| Shielding (dB) | >120 | >90 | >60-90 |
| Cost (relative) | 2-5x | 1.5-3x | 1x |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
What probe pitch should I use?
150 μm pitch for measurements above 40 GHz. 200 μm for 20-40 GHz. 250 μm for below 20 GHz. The probe pitch determines the CPW ground-signal-ground spacing at the probe contact point. Match the probe pads to the probe tip configuration exactly.
How many ground vias do I need?
Place ground vias as close to the signal via as possible, spaced no more than λ/20 apart along the transition. For a 40 GHz design on 10 mil substrate, this means vias every 5-7 mil. Use the smallest via diameter your fabricator supports for tightest via-to-signal spacing.
Can I use this transition for permanent connections?
The probe launch is designed for temporary probe contact during testing. For permanent connections, use a connector launch (SMA, 2.92mm, etc.) with a proper coax-to-microstrip transition. The probe launch is not suitable for permanent connections due to the open contact interface.